Transportation of liquids and other fluids through pipes has long been known. It is often necessary to insert a flow meter, valve or other instrument at various places along the pipeline. The couplings at these joints must be strong enough to withstand the forces exerted on the pipes and must include sealing means to prevent leakage of the fluids from the pipe. Furthermore, due to the lengths and weights of the pipes involved, it is desirable to provide couplings which permit simple insertion and removal of the instruments in the fluid flow path, without moving the pipes.
Until recently, most of the pipes used for fluid flow, and particularly for water flow, were rigid metal pipes. The conventional manner of joining a metal instrument to metal pipes is by use of a union coupling including three elements: the thread end (i.e., the outlet of the instrument to be joined which is threaded with external threads), the bottom end (a nipple member adapted to be affixed to the end of the pipe to be joined, the nipple being fitted with interior threads and a small exterior shoulder) and a nut having an inside flange at one end and an inside thread like that on the exterior of the thread end at the other end. Generally a gasket or flat seal is placed between the thread and bottom ends, which are drawn together by the nut.
The advantage of these couplings is that instruments can be inserted into the pipeline without the need to translate or rotate the existing pipeline during attachment. Nonetheless, tension is often created on the instrument inserted into the pipeline due to imprecision of the fit between adjacent elements. That is, if the sections of pipe are a little too far apart or a little too close together for the ideal fit of the instrument therebetween, pressure and tension are created on the instrument.
In addition, torque stress is often generated through the tightening of the union on the elements. And finally, differences in ambient temperature cause piping to expand and contract, and the instrument suffers from the change in tension. As a particular example, when water meters are inserted into a pipeline, the pressure on the body of the water meter due to the method of coupling can be much greater than that which the meter must withstand during operation.
With conventional metal water meters and other metal instruments, these problems are essentially negligible, since the metal is stong enough to withstand these additional forces. However, with the advent of plastic instruments, the problem of couplings is more serious. The additional tension on these instruments frequently causes the instruments to break or malfunction.